RE_FORMAT(7) RE_FORMAT(7)
NAME
re_format - POSIX 1003.2 regular expressions
DESCRIPTION
Regular expressions (``RE''s), as defined in POSIX 1003.2,
come in two forms: modern REs (roughly those of egrep;
1003.2 calls these ``extended'' REs) and obsolete REs
(roughly those of ed; 1003.2 ``basic'' REs). Obsolete REs
mostly exist for backward compatibility in some old pro-
grams; they will be discussed at the end. 1003.2 leaves
some aspects of RE syntax and semantics open; `' marks
decisions on these aspects that may not be fully portable
to other 1003.2 implementations.
A (modern) RE is one or more non-empty branches, separated
by `|'. It matches anything that matches one of the
branches.
A branch is one or more pieces, concatenated. It matches
a match for the first, followed by a match for the second,
etc.
A piece is an atom possibly followed by a single `*', `+',
`?', or bound. An atom followed by `*' matches a sequence
of 0 or more matches of the atom. An atom followed by `+'
matches a sequence of 1 or more matches of the atom. An
atom followed by `?' matches a sequence of 0 or 1 matches
of the atom.
A bound is `{' followed by an unsigned decimal integer,
possibly followed by `,' possibly followed by another
unsigned decimal integer, always followed by `}'. The
integers must lie between 0 and RE_DUP_MAX (255) inclu-
sive, and if there are two of them, the first may not
exceed the second. An atom followed by a bound containing
one integer i and no comma matches a sequence of exactly i
matches of the atom. An atom followed by a bound contain-
ing one integer i and a comma matches a sequence of i or
more matches of the atom. An atom followed by a bound
containing two integers i and j matches a sequence of i
through j (inclusive) matches of the atom.
An atom is a regular expression enclosed in `()' (matching
a match for the regular expression), an empty set of `()'
(matching the null string), a bracket expression (see
below), `.' (matching any single character), `^' (match-
ing the null string at the beginning of a line), `$'
(matching the null string at the end of a line), a `\'
followed by one of the characters `^.[$()|*+?{\' (matching
that character taken as an ordinary character), or a sin-
gle character with no other significance (matching that
character). A `{' followed by a character other than a
digit is an ordinary character, not the beginning of a
bound. It is illegal to precede a character with `\'
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other than as provided above, or to end an RE with `\'.
A bracket expression is a list of characters enclosed in
`[]'. It normally matches any single character from the
list (but see below). If the list begins with `^', it
matches any single character (but see below) not from the
rest of the list. If two characters in the list are sepa-
rated by `-', this is shorthand for the full range of
characters between those two (inclusive) in the collating
sequence, e.g. `[0-9]' in ASCII matches any decimal digit.
It is illegal for two ranges to share an endpoint, e.g.
`a-c-e'. Ranges are very collating-sequence-dependent,
and portable programs should avoid relying on them.
To include a literal `]' in the list, make it the first
character (following a possible `^'). To include a lit-
eral `-', make it the first or last character, or the sec-
ond endpoint of a range. To use a literal `-' as the
first endpoint of a range, enclose it in `[.' and `.]' to
make it a collating element (see below). With the excep-
tion of these and some combinations using `[' (see next
paragraphs), all other special characters, including `\',
lose their special significance within a bracket expres-
sion.
Within a bracket expression, a collating element (a char-
acter, a multi-character sequence that collates as if it
were a single character, or a collating-sequence name for
either) enclosed in `[.' and `.]' stands for the sequence
of characters of that collating element. The sequence is
a single element of the bracket expression's list. A
bracket expression containing a multi-character collating
element can thus match more than one character, e.g. if
the collating sequence includes a `ch' collating element,
then the RE `[[.ch.]]*c' matches the first five characters
of `chchcc'.
Within a bracket expression, a collating element enclosed
in `[=' and `=]' is an equivalence class, standing for the
sequences of characters of all collating elements equiva-
lent to that one, including itself. (If there are no
other equivalent collating elements, the treatment is as
if the enclosing delimiters were `[.' and `.]'.) For
example, if o and ^
o are the members of an equivalence
class, then `[[=o=]]', `[[=^
o=]]', and `[o^
o]' are all syn-
onymous. An equivalence class may not be an endpoint of a
range.
Within a bracket expression, the name of a character class
enclosed in `[:' and `:]' stands for the list of all char-
acters belonging to that class. Standard character class
names are:
alnum digit punct
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alpha graph space
blank lower upper
cntrl print xdigit
Which stand for the character classes defined in ctype(3).
A locale may provide others. A character class may not be
used as an endpoint of a range.
In the event that an RE could match more than one sub-
string of a given string, the RE matches the one starting
earliest in the string. If the RE could match more than
one substring starting at that point, it matches the
longest. Subexpressions also match the longest possible
substrings, subject to the constraint that the whole match
be as long as possible, with subexpressions starting ear-
lier in the RE taking priority over ones starting later.
Note that higher-level subexpressions thus take priority
over their lower-level component subexpressions.
Match lengths are measured in characters, not collating
elements. A null string is considered longer than no
match at all. For example, `bb*' matches the three middle
characters of `abbbc', `(wee|week)(knights|nights)'
matches all ten characters of `weeknights', when `(.*).*'
is matched against `abc' the parenthesized subexpression
matches all three characters, and when `(a*)*' is matched
against `bc' both the whole RE and the parenthesized
subexpression match the null string.
If case-independent matching is specified, the effect is
much as if all case distinctions had vanished from the
alphabet. When an alphabetic that exists in multiple
cases appears as an ordinary character outside a bracket
expression, it is effectively transformed into a bracket
expression containing both cases, e.g. `x' becomes `[xX]'.
When it appears inside a bracket expression, all case
counterparts of it are added to the bracket expression, so
that (e.g.) `[x]' becomes `[xX]' and `[^x]' becomes
`[^xX]'.
No particular limit is imposed on the length of REs. Pro-
grams intended to be portable should not employ REs longer
than 256 bytes, as an implementation can refuse to accept
such REs and remain POSIX-compliant.
Obsolete (``basic'') regular expressions differ in several
respects. `|', `+', and `?' are ordinary characters and
there is no equivalent for their functionality. The
delimiters for bounds are `\{' and `\}', with `{' and `}'
by themselves ordinary characters. The parentheses for
nested subexpressions are `\(' and `\)', with `(' and `)'
by themselves ordinary characters. The list of characters
that become ordinary when preceded by `\' is `^.[$*\'.
`^' is an ordinary character except at the beginning of
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the RE or the beginning of a parenthesized subexpression,
`$' is an ordinary character except at the end of the RE
or the end of a parenthesized subexpression, and `*' is an
ordinary character if it appears at the beginning of the
RE or the beginning of a parenthesized subexpression
(after a possible leading `^'). Finally, there is one new
type of atom, a back reference: `\' followed by a non-zero
decimal digit d matches the same sequence of characters
matched by the dth parenthesized subexpression (numbering
subexpressions by the positions of their opening parenthe-
ses, left to right), so that (e.g.) `\([bc]\)\1' matches
`bb' or `cc' but not `bc'.
SEE ALSO
regex(3)
POSIX 1003.2, section 2.8 (Regular Expression Notation).
BUGS
Having two kinds of REs is a botch.
The current 1003.2 spec says that `)' is an ordinary char-
acter in the absence of an unmatched `('; this was an
unintentional result of a wording error, and change is
likely. Avoid relying on it.
Back references are a dreadful botch, posing major prob-
lems for efficient implementations. They are also some-
what vaguely defined (does `a\(\(b\)*\2\)*d' match
`abbbd'?). Avoid using them.
1003.2's specification of case-independent matching is
vague. The ``one case implies all cases'' definition
given above is current consensus among implementors as to
the right interpretation.
AUTHOR
Henry Spencer
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